Cage or polyhedral oligomeric siloxanes (oligosiloxanes) are structured such that various organic groups are positioned in the periphery of an inorganic core having a size of nanometer order. Efforts have been made to develop such cage oligosiloxanes which are useful as reinforcements, fire retardants or the like when added to organic polymers. For example, U.S. Pat. No. 6,362,279 describes a method for enhancing the fire retardancy of plastics by adding cage oligosiloxanes thereto. International Publication No. 01/72885 discloses to blend a cage oligosiloxane in a polymer by melt kneading for improving mechanical properties.
Aside from simple blending of a cage oligosiloxane with a polymer, there have been reported a number of attempts to combine a polymer with a cage oligosiloxane through covalent bonds for improving mechanical, thermal and other physical properties. For example, U.S. Pat. No. 5,484,867 describes the synthesis of polymers using cage oligosiloxanes having various polymerizable groups. Also U.S. Pat. No. 6,653,365 discloses as a dental material a composition comprising a cage oligosiloxane having a reactive group and a polymerizable resin.
In these patents, monofunctional polymerizable cage oligosiloxanes are used. If a difunctional or polyfunctional monomer or monomers are present in admixture, they become an unexpected cause of crosslinking, making it difficult to control the molecular weight and physical properties of the resulting polymers. The above-referred U.S. Pat. No. 5,484,867 describes that a high purity monofunctional cage oligosiloxane is prepared by reacting a cage heptasiloxane triol (referred to as T7-trisilanol, hereinafter) with a polymerizable organotrichlorosilane (Rx is a polymerizable functional group) according to the following scheme A.

While it is inevitable to first prepare the precursor, T7-trisilanol, the type of substituent group (R1 in scheme A) on silicon with which this compound can be isolated in high yields and in high purity is limited. Typical examples are isobutyl, cyclopentyl and cyclohexyl.
However, cage oligosiloxanes having alkyl or cycloalkyl groups substituted thereon are generally low soluble. Therefore, a large volume of solvent is necessary for the polymerization reaction of polymerizable cage oligosiloxanes having such substituent groups. In the case of copolymerization with another monomer, it is difficult to increase the ratio of a cage siloxane-containing monomer.
To the solubility of cage oligosiloxane, substituent groups on the silicon atoms forming a polyhedral skeleton have a substantial contribution. For example, JP-A 6-329687 is successful in improving the compatibility of a cage oligosiloxane with organopolysiloxane by introducing linear organooligosiloxy groups on silicon atoms.
It is, however, difficult to synthesize T7-trisilanol having such solubility-enhancing substituent groups. It is thus impossible to prepare a cage oligosiloxane having polymerizable functional groups Rx by the method of scheme A.
As an alternative method, JP-A 2000-298286 describes that a polymerizable cage oligosiloxane is prepared by mixing two different organotrialkoxysilanes and subjecting them to hydrolysis and condensation, as shown by the following scheme B.

JP-A 2-178291 describes that a cage oligosiloxane having two types of organosiloxy group is obtained, through silylation of a cage ammonium silicate, by using two different silylating agents together or by effecting exchange reaction of organosiloxy groups bonded to silicon atoms forming a polyhedral skeleton. JP-A 2004-83626 and JP-A 2004-189840 disclose resin compositions comprising cage oligosiloxanes. Some cage oligosiloxanes are exemplified although the number of polymerizable groups is not specified. It is described that cage oligosiloxanes can be prepared through hydrolysis of trifunctional organosilicon monomers.
With these methods, however, the product is naturally a mixture of various siloxanes as demonstrated in Examples of JP-A 2-178291. Even if the desired monofunctional monomer is produced, its purification is difficult. It is thus difficult to obtain a monofunctional cage oligosiloxane of high purity.
There is a need for a method for preparing a polymerizable cage oligosiloxane having an increased solubility in high yields and high purity.